1. Field of the Invention
The present invention generally relates to a thin film magnetic memory device. More particularly, the present invention relates to a thin film magnetic memory device having a redundant structure.
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the use of thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells significantly improves the performance of the MRAM device. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7.2, February 2000, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000, and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 14 schematically shows the structure of a memory cell having a magnetic tunnel junction (hereinafter, sometimes simply referred to as “MTJ memory cell”).
Referring to FIG. 14, the MTJ memory cell includes a tunneling magneto-resistance element TMR having its electric resistance varying according to the storage data level, and an access element ATR for forming a path of a sense current Is that flows through tunneling magneto-resistance element TMR in data read operation. Access element ATR is typically formed from a field effect transistor. Therefore, access element ATR is hereinafter sometimes referred to as access transistor ATR. Access transistor ATR is coupled between the tunneling magneto-resistance element TMR and a fixed voltage (ground voltage Vss).
For the MTJ memory cell are provided a write word line WWL for data write operation, a read word line RWL for data read operation, and a bit line BL. Bit line BL serves as a data line for transmitting an electric signal corresponding to the storage data level in both data read and write operations.
FIG. 15 is a conceptual diagram illustrating data read operation from the MTJ memory cell.
Referring to FIG. 15, tunneling magneto-resistance element TMR has a ferromagnetic material layer FL having a fixed magnetization direction (hereinafter, sometimes simply referred to as “fixed magnetic layer”), and a ferromagnetic material layer VL that is magnetized in the direction according to an external magnetic field (hereinafter, sometimes simply referred to as “free magnetic layer”). A tunneling barrier (tunneling film) TB of an insulator film is interposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized either in the direction that is the same as or opposite to that of fixed magnetic layer FL according to the write data level. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunnel junction.
In data read operation, access transistor ATR is turned ON in response to activation of read word line RWL. This allows sense current Is to flow through a current path formed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR and ground voltage Vss.
The electric resistance of tunneling magneto-resistance element TMR varies according to the relation between the respective magnetization directions of fixed magnetic layer FL and free magnetic layer VL. More specifically, when fixed magnetic layer FL and free magnetic layer VL have the same (parallel) magnetization direction, tunneling magneto-resistance element TMR has a lower electric resistance as compared to the case where they have opposite (antiparallel) magnetization directions.
Accordingly, when free magnetic layer VL is magnetized in the direction according to the storage data, a voltage change produced at tunneling magneto-resistance element TMR by sense current Is varies according to the storage data level. Therefore, applying sense current Is to tunneling magneto-resistance element TMR after precharging bit line BL to a fixed voltage would enable the storage data in the MTJ memory cell to be read by sensing the voltage on bit line BL.
FIG. 16 is a conceptual diagram illustrating data write operation to the MTJ memory cell.
Referring to FIG. 16, in data write operation, read word line RWL is inactivated, and access transistor ATR is turned OFF. In this state, a data write current for magnetizing free magnetic layer VL in the direction according to the write data is applied to write word line WWL and bit line BL. The magnetization direction of free magnetic layer VL is determined by the respective data write currents flowing through write word line WWL and bit line BL.
FIG. 17 is a conceptual diagram illustrating the relation between the data write current and the magnetization direction of the tunneling magneto-resistance element in the data write operation to the MTJ memory cell.
Referring to FIG. 17, the abscissa H(EA) indicates a magnetic field that is applied in the easy axis (EA) direction in free magnetic layer VL of tunneling magneto-resistance element TMR. The ordinate H(HA) indicates a magnetic field that is applied in the hard axis (HA) direction in free magnetic layer VL. Magnetic fields H(EA), H(HA) respectively correspond to two magnetic fields produced by the currents flowing through bit line BL and write word line WWL.
In the MTJ memory cell, fixed magnetic layer FL is magnetized in the fixed direction along the easy axis of free magnetic layer VL. Moreover, free magnetic layer VL is magnetized either in the same (parallel) direction as, or in the opposite (antiparallel) direction to, that of fixed magnetic layer FL along the easy axis, according to the storage data level (“1” and “0”). In the specification, it is assumed that tunneling magneto-resistance element TMR has electric resistance values Rmax and Rmin (Rmax>Rmin) respectively corresponding to the above two magnetization directions of free magnetic layer VL. The MTJ memory cell is thus capable of storing 1-bit data (“1” and “0”) corresponding to the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only when the sum of the applied magnetic fields H(EA) and H(HA) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetization direction of free magnetic layer VL will not change if an applied data write magnetic field corresponds to the region inside the asteroid characteristic line.
As shown by the asteroid characteristic line, applying a magnetic field of the hard axis direction to free magnetic layer VL enables reduction in a magnetization threshold value required to change the magnetization direction along the easy axis.
When the operation point of the data write operation is designed as in the example of FIG. 17, a data write magnetic field of the easy axis direction is designed to have a strength HWR in the MTJ memory cell to be written. In other words, the value of the data write current to be applied across bit line BL or write word line WWL is designed to produce data write magnetic field HWR. In general, data write magnetic field HWR is defined by the sum of a switching magnetic field HSW required to switch the magnetization direction and a margin ΔH. Data write magnetic field HWR is thus defined by HWR=HSW+ΔH.
In order to rewrite the storage data of the MTJ memory cell, that is, the magnetization direction of tunneling magneto-resistance element TMR, a data write current of at least a prescribed level must be applied to both write word line WWL and bit line BL. Free magnetic layer VL in tunneling magneto-resistance element TMR is thus magnetized either in the same (parallel) direction as, or in the opposite (antiparallel) direction to, that of fixed magnetic layer FL according to the direction of the data write magnetic field along the easy axis (EA). The magnetization direction written to tunneling magneto-resistance element TMR, i.e., the storage data of the MTJ memory cell, is held in a non-volatile manner until another data write operation is conducted.
The electric resistance of tunneling magneto-resistance element TMR thus varies according to the magnetization direction that is rewritable by an applied data write magnetic field. Accordingly, non-volatile data storage can be realized by using the two magnetization directions of free magnetic layer VL of tunneling magneto-resistance element TMR as the respective storage data levels (“0” and “1”).
In order to improve the manufacturing yield, a memory device commonly includes not only a plurality of normal memory cells that are selected according to an address signal but also a redundant structure for replacing a defective normal memory cell, if any.
In such a redundant structure, a defective memory cell is replaced with a spare memory on a section-by-section basis. Accordingly, for the memory device having the redundant structure, whether or not an input address signal matches a fault address designating a defective memory cell must be determined in both data read and write operations. Even the MRAM device having the redundant structure must be designed so that the time required for such determination does not significantly affect the operation speed.
In order to write data to the MTJ memory cell in the MRAM device, that is, in order to rewrite the magnetization direction of the tunneling magneto-resistance element, the data write magnetic fields of two directions are applied as described in FIG. 17. Accordingly, if the data write magnetic fields do not properly change with time, the operation of magnetizing the tunneling magneto-resistance element is destabilized, thereby possibly resulting in erroneous write operation.